I will be in Germany in a few days an am going to take my Nexus One with me. I want to get a pre-paid card, for DATA only. I wont be making any calls, but will be using Data for text (Google Voice) and for GPS (Google Maps).

Any recommendations on what my best option would be?

I was considering Vodafone, but I dont really know. I saw their plans were 1 euro a day for 5mb of DATA... that's not very much!

I'm not sure how much data I'll be using. I think Lidl's 15 euro a month for 5GB is pretty good.

I'm not sure about getting a different carrier for each country. I'm only going to be in Switzerland for 3 days... I dont want to pay 5 francs for a SIM card and then load it up with 10 francs, only to use 3 of them.

I'm going to be in Austria for 3 days as well. Do I really need to get a new SIM card from a different carrier? Does Lidl not offer roaming capabilities?

Data roaming with Lidl would cost € 1.50/MB in Austria and € 12.00/MB in Switzerland. Given these rates a separate SIM for each country would quickly pay off.

If you activated the 500MB data pack with Lidl you would automatically benefit from the "EU Internet-Tages-Pack" which applies in EU countries (so not in Switzerland as it is no part of the EU) and gives you 10MB for € 3/day (afterwards you get disconnected until midnight).
Unfortunately this "EU Internet-Tages-Pack" is only available in combination with the 500MB-pack. If you activate any other data option (even the bigger 5GB pack) you will be stuck to the volume-based tariffs given above.